1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, and more particularly to an electrophotographic photosensitive member comprising a low-molecular organic photoconductive material that can impart improved electrophotographic characteristics.
2. Related Background Art
Inorganic photoconductive materials such as selenium, cadmium sulfide and zinc sulfide have been hitherto known as photoconductive materials used in electrophotographic photosensitive members. These photoconductive materials have a number of advantages, for example, that they can be electrostatically charged to a suitable potential in the dark, cause less charge loss in the dark, and can rapidly release charges as a result of irradiation by light, but, on the other hand, have various disadvantages. For example, in selenium photosensitive members, they may be readily crystallized by the factors such as temperature, humidity, dust and pressure, and, in particular, exceedingly crystallized at an atmospheric temperature more than 40.degree. C., resulting in a lowering of chargeability, or generation of white dotts on an image. In cadmium sulfide photosensitive members, no stable sensitivity can be obtained in a highly humid environment. In zinc oxide photosensitive members, they require a sensitization effect attributable to sensitizing coloring matters as typified by Rose Bengale, but, since such sensitizing coloring matters bring about charge deterioration caused by corona charging or color-fading by light, can not provide stable images over a long period of time, disadvantageously.
On the other hand, various types of organic photoconductive polymers including polyvinyl carbazole have been hitherto proposed. These polymers, however, have been put into practical use with difficulty up to the present, notwithstanding their superiority to inorganic photoconductive materials in respect of film-forming properties and lightness in weight. This is because they have achieved no sufficient film-forming properties and also are inferior to the inorganic photoconductive materials in respect of sensitivity, durability, and stability against environmental changes. Also proposed are low-molecular organic photoconductive materials such as hydrazone compounds as disclosed in U.S. Pat. No. 4,150,987, triarylpyrazoline compounds as disclosed in U.S. Pat. No. 3,837,851, and 9-styryl anthracene compounds as disclosed in Japanese Patent Laid-open Application No. 51-94828 and Japanese Patent Laid-open Application No. 51-94829. Such low-molecular organic photoconductive materials have become able to eliminate the disadvantage in film-forming properties that has been questioned in the field of organic photoconductive polymers, by appropriately selecting binders to be used. They, however, can not be said to be satisfactory in respect of the sensitivity.
Under such circumstances, a laminated structure comprising a photosensitive layer functionally separated into a charge generation layer and a charge transport layer has been proposed in recent years. Electrophotographic photosensitive members employing this laminated structure as the photosensitive layer have become able to improve the sensitivity, charge retension, surface strength, etc. to visible light. Such electrophotographic photosensitive members are disclosed, for example, in U.S. Pat. Nos. 3,837,851 and 3,871,882.
However, in the electrophotographic photosensitive members employing the conventional low-molecular organic photoconductive material in the charge transport layer, the sensitivity is not necessarily satisfactory, and, in particular, light portion potential and dark portion potential may greatly fluctuate when charging and exposure to light are repeatedly carried out. Thus, there is a room for improvement.